The invention relates to a high-pressure metal halide discharge lamp provided with a light-transmitting lamp vessel which is sealed in a vacuumtight manner and contains an ionizable filling with rare gas and metal halide, and in which tungsten electrodes are arranged connected to current conductors which issue to the exterior through the lamp vessel, which electrodes are provided with an oxidic electron emitter.
Such a high-pressure metal halide discharge lamp is known from U.S. Pat. No. 4,574,219.
Near their free ends, the electrodes of the known lamp are provided with, for example coated with, a cermet of tungsten and metal oxide chosen from the oxides of scandium, aluminium, dysprosium, thorium, yttrium, and zirconium and mixtures thereof. The cermet in this case comprises 2 to 30% by weight metal oxide.
It is the object of these electrodes to render it possible that the lamp quickly enters its operational state after starting and that a preceding period of a glow discharge is avoided. For this purpose, the cermet is porous so that the electrodes have a low thermal conductivity and consequently quickly assume their operational temperature.
The complicated structure of the electrodes and the resulting complicated manufacture of the electrodes constitute a disadvantage. Another disadvantage of the known lamp is the use of the radioactive thorium oxide. This represents a severe strain on the environment, both during its manufacture and during manufacture of the electrodes, and also at the end of lamp life. Another disadvantage is that the emitter is comparatively quickly exhausted when oxides other than thorium oxide are used.
Emitter is usually present in or on electrodes in discharge lamps for facilitating the emission of electrons. In proportion as the emitter has a lower work function compared with the electrode material without emitter the electrode will assume a lower temperature during operation. The evaporation of electrode material and deposition of the vapour on the lamp vessel are smaller then. A result of this is that the lamp has a higher luminous maintenance: its initial luminous efficacy (lm/W) is better maintained during lamp life. It is in particular the noxious thorium oxide which has a low work function.
EP 0,136,726-A2 discloses a high-pressure sodium discharge lamp in which similar oxidic materials are used as emitters. In or on the electrodes there are present one or several of the oxides of yttrium, lanthanum, cerium, hafnium, thorium, beryllium and scandium. These oxides are more stable than BaO which is sometimes used as an emitter in high-pressure sodium discharge lamps, and are accordingly supposed to counteract the loss of sodium from the lamp vessel.
U.S. Pat. No. 3,700,951 discloses high-pressure sodium and high-pressure mercury discharge lamps which have refractory electrodes with an emitter arranged in a cylinder at the free end of each of these electrodes, which emitter is made of tungsten, molybdenum or tantalum with a first metal chosen from the lanthanides and thorium and a second metal chosen from elements having atomic numbers 22 to 28, 44 to 46 and 76 to 78, the alloy of said first and second metal moistening the tungsten, molybdenum or tantalum. These lamps have similar disadvantages as does the lamp mentioned first.
U.S. Pat. No. 4,303,848 discloses a high-pressure discharge lamp in which a sintered body has been placed on an electrode rod of tungsten, which body is built up from tungsten, molybdenum, tantalum, and mixtures thereof, with an oxide of yttrium, zirconium, aluminium and mixtures thereof, and with an alkaline earth compound serving as the emitter. The purpose of the oxide here is to replace thorium oxide in preventing contact between the alkaline earth compound and the metal. Therefore, comparatively large quantities of oxide of up to 30% by weight are used.